Recently, there have been efforts to develop systems to help in the rehabilitation of people who have suffered an injury or disability (for example due to a stroke), and in particular to provide systems that can instruct the user to perform particular exercises, monitor the movements of the different parts of the user's body and provide useful feedback to the user on their movements, without requiring a physiotherapist or other professional to be present. Of course, these systems can also be used in the presence of a physiotherapist or other professional to help them provide effective therapy to the user.
Typically, users will spend a lot of time practicing basic exercises. This leads to frustration for the users because it is difficult to see the relation between the basic exercises that they need to practice and the activities of daily life that they want to recover. Without seeing this connection, users can become de-motivated in practicing the basic exercises.
In order to motivate users to perform basic exercises, the user should understand the relation between the basic exercises and the final goal. Users that have suffered a stroke often have cognitive difficulties as well as physical problems, so it is desirable to present the link between basic exercises and final goals in an intuitive way.
Feedback and instructions to the user can be provided, at least in part, by a graphical representation of the user on a display device. This graphical representation can provide a computer-generated image of the user, so that the user can see whether their movement and posture is correct. The graphical representations provide the advantage that it is possible for the user to see their own movements from different view points (for example, the graphical representation can be a mirror image, a true (non-mirrored) image, a view from the side, etc.). These graphical representations are often known as avatars.
Parts of the user's body can be monitored by respective sensor units that include motion sensors (such as accelerometers, magnetometers and gyroscopes) that measure the position and motion of the part of the body in a world coordinate frame of reference. Normally at least five sensor units are required, attached, respectively, to the chest and upper and lower arms. This allows the avatar to represent the movement and posture of the upper half of the user's body.
Additional sensor units can be attached to the legs to allow the avatar to represent the whole of the user's body. Clearly, the more sensor units that are placed on the user's body, the more accurate the avatar can be.
However, a problem arises in that the algorithm that creates the graphical representation of the user from the sensor unit data has no knowledge of the orientation or position of the display device, which means that it is difficult to use the display device as, say, a virtual mirror (so that when the user faces the display device, the graphical representation of the user faces the user).
If no action is taken, this desired situation is only reached for a single arrangement of the display device. FIG. 1 shows an example of this particular arrangement. Here, the user 2 is facing a display device 4. The user is facing north, and the display screen 5 of the display device 4 is oriented along an east-west axis, with the display screen 5 facing south. A number of sensor units 6 are attached to the user 2 for measuring the position and motion of the user 2.
The algorithm that creates the graphical representation 8 is configured so that the graphical representation 8 faces out of the display device 4 when the user 2 is facing north (as measured by the magnetometer(s) in the sensor units 6).
However, as shown in FIG. 2, if the display device 4 is not oriented along an east-west axis, the graphical representation 8 created using the same algorithm will not be a mirror image of the user 2.
In particular, the display device 4 is oriented along a north-south axis with the display screen 5 of the display device 4 facing west. As the user 2 is now facing east, the algorithm creates the graphical representation 8 that is turned to the left on the display screen 5 (i.e. facing south).
This problem results from the orientation of the user 2 being measured in a world-fixed frame of reference by the magnetometers (otherwise known as electronic compasses) in the sensor units 6.
One approach to get around this problem is to provide a control for setting the compass rotation of the graphical representation 8 manually.
Another option is to calibrate the algorithm with respect to the orientation or position of the display device 4. Typically, this is done by getting the user 2 to face the display device 4, and using the orientation (magnetometer) measurement from the sensor units 6 to calibrate the algorithm. Only after this initial measurement is taken can the graphical representation 8 be correctly displayed on the display screen 5.
However, it is desirable to provide a solution to this problem that does not require manual action or calibration by the user.